Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
A variable wind speed turbine may typically be controlled by varying the generator torque and the pitch angle of the blades. As a result, aerodynamic torque, rotor speed and electrical power will vary.
A common control strategy of a variable speed wind turbine is described with reference to FIG. 1. In FIG. 1, the operation of the wind turbine is illustrated in terms of the pitch angle (β), the electrical power generated (P), the generator torque (M) and the rotational velocity of the rotor (ω), as a function of the wind speed.
In a first operational range, just above the cut-in wind speed, the rotor is controlled to rotate at a substantially constant speed that is just high enough to be able to accurately control it. The cut-in wind speed may be e.g. 3 m/s.
In a second operational range, the objective is generally to maximize power output by maintaining the pitch angle of the blades constant so as to capture maximum energy. The generator torque and rotor speed are varied so as to keep the tip speed ratio (tangential velocity of the tip of the rotor blades divided by the prevailing wind speed) constant so as to maximize the power coefficient Cp.
In general, the power coefficient of a wind turbine may be calculated in accordance with the following equation:
            C      P        =                  P        captured                    P        avaiable              ,wherein
Pcaptured is the aerodynamic power captured by the rotor, and Pavailable is the power available in the wind passing through the rotor swept area. The power coefficient Cp is thus a measure of how efficient the wind turbine is operating.
The available power in the wind may be calculated in accordance with the following equation:
            P      available        =                  ρ        ·                  V          3                ·        A            2        ,wherein
ρ is the air density, A is the rotor swept area and V is the wind speed, which in this equation is assumed to be constant over the entire rotor diameter.
In order to maximize power output and keep Cp constant at its maximum value, the rotor torque may be set approximately in accordance with the following equation:T=κ·ω2, whereink is a constant, and ω is the rotational speed of the generator. In a direct drive wind turbine, the generator speed substantially equals the rotor speed. In a wind turbine comprising a gearbox, normally, a substantially constant ratio exists between the rotor speed and the generator speed.
In a third operational range, which starts at reaching nominal rotor rotational speed and extends until reaching nominal power, The rotor speed is kept constant, and the generator torque is varied to such effect. This may correspond to a wind speed range of e.g. approximately 8.5 m/s to approximately 11 m/s.
In a fourth operational range, above the nominal wind speed to the cut-out wind speed (for example from approximately 11 m/s to 25 m/s), the blades are rotated (“pitched”) to maintain the aerodynamic torque delivered by the rotor substantially constant. At the cut-out wind speed, the wind turbine's operation is interrupted.
In the first, second and third operational ranges, i.e. at wind speeds below the nominal wind speed, the blades are kept in a constant pitch position, namely the “below rated pitch position” at wind speeds equal to or below nominal wind speed. Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions depends however on the complete design of the wind turbine.
Depending on the precise location of a wind turbine, and the corresponding local wind regime, the occurrences of high speed winds and low speeds winds may vary. Generally speaking however, the wind turbine spends most of its operating life in the second operation range, i.e. the range of theoretical optimum power coefficient. It is thus important, that especially during this second operational range the wind turbine yields as much energy as possible.
The performance of wind turbines is generally assessed off-line, i.e. not during the actual operation of the wind turbine but by the application of off-line statistical methods to extract information from a large number of parameters that have been measured and/or registered during operation. Alternatively, power curve measurements to characterize the performance of wind turbines over the complete operating range may be performed under standardized circumstances. In the latter case, large periods of time are usually required for this kind of testing as said standardized circumstances might be difficult to obtain due to the inherent stochastic behaviour of the wind.
Power curve measurements may also be performed based on measurements of an anemometer arranged on the nacelle. It is further known to compare power curves of one wind turbine with another turbine operating in substantially the same wind regime. The measurements of the anemometer on the nacelle are easily available however relatively inaccurate; the flow of the air is generally so disturbed by the rotor that the wind speed may not be representative for the wind speed in the rotor swept area. Furthermore, especially in the case of wind turbines with rotors having larger diameters, the wind speed may vary significantly over the rotor diameter. In order to improve the accuracy of nacelle anemometry, calibration procedures may be performed.
Generally speaking, off-line methods used for wind turbine characterization can indeed provide relevant information about its long-term operational state. Nevertheless, they are usually quite time-consuming, so they lack the ability to provide fast responses to the system. Furthermore, statistical analysis can fail to detect specific operating regimes, especially those with low probability of occurrence, which can be smoothed and remain undetected after statistical treatment of data.
There may be various causes that can cause a deterioration of the performance of a wind turbine during its operation, such as e.g. dirt accretion on the blades, wind shear, yaw misalignment, drive train deterioration, control errors etc. It is thus important that the performance of a wind turbine can be monitored in real-time during operation of the turbine.
In various examples of the present invention, such monitoring is provided. Furthermore, with the proper monitoring, the operation of the wind turbine may be adjusted to improve the energy yield if inefficient operation is detected.